In order for cancers to progress, metastasize, and become treatment refractory, gene expression programs must be modulated. Transcriptional programs generate transcripts within the cells, while post-transcriptional programs regulate the half-lives, localization, and translational efficiency of cellular transcripts. Such post-transcriptional deregulation has been implicated as a major means by which ?aggressive? gene expression progression are established. We recently showed that RNA-fragments (tRF?s) generated from the processing of specific tRNA molecules suppress breast cancer metastasis through their binding and repression of an oncogenic RNA-binding protein (Goodarzi et al., Cell, 2015). A search for a tRF that could promote cancer progression has led to our identification of a specific tRNA-fragment (tRF) that is increased in highly metastatic breast cancer cells, promotes metastasis, and suppresses expression of transcripts containing its recognition motif. This mode of regulation contrasts RNAi-based mechanisms, since the modulated transcripts contain sense (rather than complementary) sequences relative to the tRF. We hypothesize that this tRF drives metastatic progression by binding and inhibiting an RNA-binding protein (RBP) that otherwise promotes the stability of pro-metastatic transcripts. We aim to investigate the role of this tRF in metastasis formation and progression; to identify the trans-factor (RBP) that this tRF regulates and the downstream regulon impacted; to uncover the upstream mechanism of tRF generation; to investigate its diagnostic potential as a predictive biomarker; and to provide proof-of-concept support for tRF therapeutic inhibition through oligonucleotide anti-sense administration. We will utilize cutting-edge, complementary, and mutli-disciplinary methods to achieve these goals. Successful completion of this study will generate new basic insights into post-transcriptional regulation by a tRNA-derived fragment, reveal how specificity is achieved in tRNA-fragment generation, achieve proof-of-concept for oligonucleotide-based therapeutic inhibition of a tRF in cancer, identify a potentially druggable tRF-producing ribonuclease, and establish diagnostic and prognostic potential for a tRF. As such, this work has significant potential for impacting human health. Our lab has made major mechanistic inroads into the non-canonical roles of tRNAs in gene-expression control in cancer (Goodarzi, Hoang et al., Cell, 2016; Goodarzi et al., Cell, 2015). Moreover, our interdisciplinary approach that integrates molecular, biochemical, genetic, computational, animal modeling, and clinical association methods to investigate basic mechanistic questions of relevance to human cancer position us well for success in these efforts.